I think that Chihayafuru is one of the most delightful anime's on the site and is over-looked by the vast majority of potential viewers due to how niche it is. I personally had no interest in it and over-looked it for weeks until I was bored one day and tried it on a whim. And man it's ridiculously good.

I agree with Haibane Renmei, but here are some other less-mentioned series:

There's Magical Mew Mew Taruto, which got a DVD release, but that's it and seems buried. I call it is "the other magical catgirl anime".
There is also Night Wizard and Iriya no Sora, UFO no Natsu, which were never even licensed in North America and might be worth a shot in a few people's book (Night Wizard if only because it's one of the few anime based off of a tabletop RPG).

Edit: Somehow I forgot Mahoraba: Heartful Days in the "unlicensed" group. I thought that was a really good series as well.

Chihayafuru
Deadman Wonderland I think would be if there was a season 2 anime production. Haven't had luck finding the manga.
Two others from years past (I don't know how their reception was): Chrome Shelled Regios and Freezing (wish there was a second season and that manga was licensed for the US).